This invention relates to a process in which various diacetylenic compounds are utilized to indicate via irreversible color transformations whether perishable commodities such as frozen foodstuffs, pharmaceuticals, vaccines etc. have been exposed to temperatures above a critical temperature which result in substantial degradation of such products.
U.S. Pat. No. 4,215,208 (Yee et al., 1980) discloses that certain diacetylenic compounds may be polymerized via radiation, and that the resulting polymers are capable of undergoing reversible color changes with changing temperatures. In regard to the above-referenced patent, Yee et al. describes a TEMPERATURE-INDICATOR DEVICE UTILIZING THERMOCHROMIC POLYACETYLENES which functioned most effectively in the temperature region of 120.degree.-200.degree. C.
U.S. application Ser. No. 938,292, filed Aug. 30, 1978, by G. N. Patel and D. M. Ivory now U.S. Pat. No. 4,439,346 (Mar. 27, 1984) (see also U.S. Pat. No. 4,452,995 issued June 5, 1984 on a division thereof) discloses that mixtures of polydiacetylenes and gel forming liquids may be utilized to construct a clinical thermometer which undergoes a color transformation when a predetermined temperature has been reached. The composition reverts to the original color upon cooling to a temperature substantially below the original temperature color transformation point. The process described by Patel and Ivory involves polymerizing a diacetylenic monomer via radiation, extracting any unreacted monomer from the polymerized material with an organic solvent, and finally obtaining the color transformation by heating the polymeric gel solution to the desired temperature which is a function of the particular components of the polymeric gel solution.
U.S. Pat. No. 4,195,055 (Patel, 1980) describes a device which measures the time-temperature exposure of perishable articles. The device utilizes a process wherein solutions of various color responsive materials (dyes, pigments, partially polymerized diacetylenes etc.) are deposited upon a substrate followed by subsequent evaporation of the solvent, and then contacting the substrate with a vapor. Upon contact with said vapor, the color responsive material undergoes a color change and simultaneously creates a moving boundary which advances along a diffusion strip as a function of time and temperature. Various modifications of the device are described in the following patents assigned to Allied Chemical Corporation: U.S. Pat. Nos. 4,195,056 (1980); 4,195,057 (1980); 4,195,058 (1980).
U.S. Pat. No. 3,615,719 (Michel et al., 1971) describes a process for determining whether frozen foods have been exposed to cumulative time-temperature exposures which result in spoilage of frozen products. The above-referenced process involves freezing an indicating liquid which is separated from a color indicating substrate (absorbent material such as blotting paper) by an intermediate layer composed of egg white, albumin, glue, gelatin, or molten sugar of a glassy consistency. Upon thawing, the indicating liquid begins to dissolve the intermediate layer, and when said intermediate layer is completely dissolved the thawed liquid interacts with the substrate causing color development of the substrate or a color change in the substrate. The liquid indicators used in the process are self-dyeing staining reagents or colorless solutions with dissolved reagents which react with reagents on the substrate to develop color or to effect the color transformation of the substrate.